Ooooh. If you have AS3 stuff already it’s easy.
You can use an Entity or a Graphic, it doesn’t really matter.
What I’d do is (pardon my vocabulary, not using Spine specifics here, maybe they call them another way), first of all extend Graphiclist to make a Spine sprite class (the one which would include the animations and everything). Then, you can add an Image to this list for each “part” of the Spine sprite, and you change the image properties for each Spine frame, according to the instructions you can read there.
So, you’d have to build a frame reader and keep track of each image to modify it, according to the animation instructions.
You could base your implementation on the AS3 runtime code you said. Instead of using the Bitmap class or the Sprite class, you’d use FlashPunk Images on your Graphiclist.
If you need more specific help, you can ask more questions indeed, and post your code if you need me to take a look. I’ll be happy to help you!